When a device wants to know the MAC address for a given IP address, it sends a packet to the broadcast MAC address asking “Who has IP address y.y.y.y?” All devices receive that packet, but only the one with the IP address y.y.y.y will respond with a packet “It’s me.” The asking device receives the answer and now knows that the source MAC address is the right MAC address to use. Once the sender has retrieved the MAC address of the next hop, he writes that target MAC address into the packet and sends the packet.ĪRP itself is a protocol above ethernet, like IP or IPX. There is a special protocol ARP (address resolution protocol) that is used for that. Since ethernet uses MAC addresses, the sender needs to get the MAC address of the next hop. Up to now things seem to have gotten worse, because now we have two IP addresses: one is the original IP packet’s target address, the other is the IP of the device to which we should send the packet (the next hop, either the final destination or the router). If x.x.x.x is in the same network, then the destination IP can be reached directly, otherwise the packet needs to be sent to the configured router. When your computer wants to send a packet to some IP address x.x.x.x, then the first check is if the destination address is in the same IP network as the computer itself. Another protocol for example would be IPX. IP is a protocol that is used on a layer above ethernet. How do IP addresses and MAC addresses work together? There are special MAC addresses, one for example is ff:ff:ff:ff:ff:ff, which is the broadcast address and addresses every network adapter in the network. If the addresses match, the packet is processed, otherwise it is discarded. If a network adapter is receiving a packet, it is comparing the packet’s destination MAC address to the adapter’s own MAC address. Packets that are sent on the ethernet are always coming from a MAC address and sent to a MAC address. Network cards each have a unique MAC address. MAC addresses are the low level basics that make your ethernet based network work. SuperUser contributor Werner Henze offers some insight into the function of the MAC address: Where indeed? What is the specific function of the MAC address? The Answer So my question is, where exactly does a MAC address come into play during a packet transfer? I don’t think it sits there for no reason.
So, MAC addresses are not used for packet transfer. With MAC addresses, there is no hierarchy, and thus packet forwarding would not be possible. I understand that IP addresses are hierarchical, so that routers throughout the internet know which direction to forward a packet.
SuperUser reader Vishnu Vivek is curious about MAC addresses and their function:
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